The main coloring component of hair is a dark pigment, melanin, which occurs as granules embedded in the hair cortex. The aim of bleaching is to decolorize selectively the pigments in the hair with minimal damage to the hair matrix. When hair is bleached, the color changes to lighter and lighter shades depending upon the amount of melanin granules dissolved and removed from the hair fiber. More specifically, bleaching occurs in two steps: (1) initial solubilization of the color granules and (2) decolorization of the dark brown solubilized pigment. The reaction between melano-protein and oxidizing compound is confined mainly to the protein-combined cystene residues which are subsequently converted to combined cysteic acid. The solubilization of the melanin granules is connected with the splitting of the disulfide bridges in the melano protein.
The bleaching process can be halted at any point or can be permitted to continue to a light blonde or platinum shade. The latter provides a good background for a variety of tints which can be obtained by a subsequent coloring step. Such bleaching and coloring combination is known as a double process coloring and causes hair damage by promoting porosity, brittleness, dryness and loss of tensile strength. Problems are encountered when the consistency of the dye or bleach preparation is too low, particularly the problem of the dye or bleach preparation running along the hair and causing undesired dyeing or bleaching of certain parts of the hair.
It has long been known that alkali-soluble acrylate dispersions may be used for thickening aqueous systems. U.S. Pat. No. 4,685,931 discloses an aqueous dispersion of a copolymer of ethyl acrylate, methacrylic acid, acrylic acid and a polyethylenically unsaturated copolymerizable monomer. The polyethylenically unsaturated copolymerizable monomer increases the molecular weight of the copolymers and hence their thickening effect. These known copolymer dispersions, however, do not completely satisfy the stringent demands needed for effective thickening, particularly when the dispersions are used in cosmetic preparations. Dispersions of these copolymers, for example, have a marked tendency towards viscosity drift. Additionally, some dispersions show only moderate thickening power; while with other dispersions, the viscosities of the thickened preparations are extremely sensitive to other ingredients in the ultimate formulation.
Particular stringent demands are made of alkali soluble acrylate copolymer dispersions used for thickening hydrogen peroxide preparations. Preparations of this type are commonly employed as developer compositions for oxidative hair dyes and for hair bleaches. The mildly acidic preparations are mixed with an alkaline dye cream just before application to the hair to form an alkaline dye preparation. Since this destroys the consistency of the cream, a thickener dispersion is included in the developer preparation in order to achieve a mixture viscosity suitable for applying the alkaline dye preparation to hair after the two components have been combined.
Alkali soluble carboxyl-group containing polymers have been used for thickening emulsions. U.S. Pat. No. 4,927,627 discloses a oil-in-water emulsion which contains a oil component, an emulsifier, hydrogen peroxide and a carboxyl-group containing polymer. Such carboxyl-group containing polymer dispersions, however, have a marked tendency towards viscosity drift.
In contrast, the present invention has overcome the above deficiencies without resorting to thickening agents, by providing a stable emulsion containing an oxidizing agent which maintains a viscosity suitable for uniform application to hair. In addition, the emulsion of the present invention, conditions the hair during processing and is prepared at room temperature. The present inventors have unexpectedly discovered that the incorporation of a food grade emulsifier into an emulsion used as the developer composition for oxidative hair dyes and for hair bleaches serves a dual function of stabilizing the viscosity as well as emulsification.